Rotary screw wheel device



ROTARY SCREW WHEEL DEVICE Filed Aug. 12, 1943 helix angle working spaceschange in volume after passing Patented Dec. 28, 1948 ROTARY SCREW WHEELDEVICE Alf Lysholm, Stockholm, Sweden. assignor. by I mesne assignments.to Jarvis 0. Marble, New York, N. Y.. Leslie M. Merrill, Westiield, N.J., and Percy H. Batten, Racine, Wls., as trustees Application August12, 1943, Serial No. 498,321

'1 Claims. 1

The present invention relates to rotary screw wheel devices of the kindin which intermeshing rotors having helical lands and grooves cooperatewith each other and with an enclosing casing to form working spaces orchambers which vary in volume as the rotors revolve. More particularly,the invention relates to such devices in which the size and location ofthe inlet and outlet ports are so related to the number, length and ofthe lands and grooves that the out of communication with the inlet portand before coming into communication with the outlet port. Still moreparticularly, the invention relates to such devices of the typedisclosed in my prior Patents Nos. 2,174,522 and 2,243,874, grantedOctober 3, 1939, and June 3, 1941, respectively.- especially but notnecessarily when operated as compressors.

Devices of the character under consideration are ordinarily operated athigh peripheral speeds with a small clearance or space packing betweeenthe peripheries of the rotors and the interior of the casings and withthe rotors geared together by suitable timing or synchronizing gears tomaintain clearance between the walls of the intermeshlng lands andgrooves. Operation at such high speeds, which are required in order toreduce toan acceptable percentage the losses due to leakage through thespace packing, involves numerous forms of dynamic losses, which must beminimized if acceptable volumetric and adiabatic efiiciencies are to beattained.

Heretofore much attention has been devoted to port locations andconfiguration in order to reduce such losses but there previously hasapparently been a lack of appreciation of the existence and magnitude ofcertain losses which it is the general object of the present inventionto reduce, nor has there been an appreciation of the manner in whichsuch losses can most effectively be reduced.

The losses with which the present invention is concerned are reductionlosses, that is, the losses incident to the how of fluid through andfrom the inlet passage into the rapidly moving grooves of the rotors,and in order to minimize such losses this invention contemplates theprovision of novel rotor structure, preferably combined with inletporting constructed and arranged in accordance with certain principleshereinafter more fully pointed out.

Since the invention is particularly advanta-- geous in its use whenapplied to compressors oi the type disclosed in my aforementionedpatents,

it will be described herein, by way of example but without limitation,as incorporated in such a device, and for a better understanding of themore detailed nature and objects of the invention, reference may best behad to the ensuing portion of this specification, taken in conjunctionwith the accompanying drawings, in which suitable embodiments of theinvention are described.

In the drawings:

Fig. 1 is an end view taken on the line l--l of Fig. 2 of a deviceembodying the invention;

Fig. 2 is a side view partlydn section and partly in elevation of thedevice seen in Fig. 1;

Figs. 3 and 4 are fragmentary sections taken on the respectivelynumbered section lines of Fig. 1;

Fig. 5 is a fragmentary end view showing a different form of one of therotors shown in Fig. 1;

Figs. 6 and 7 are fragmentary sections taken on the respectivelynumbered section lines of Fig. 1;

Fig. 8 is a fragmentary view similar to Fig. 5 showing another form ofone of the rotors shown in Fig. 1; and

Figs. 9 and 10 are fragmentary views showing respectively still otherforms of rotors of the kind shown in Fig. 1.

Referring now more "particularly to Figs. 1 and 2, the compressorcomprises a housing consisting of a central barrel portion I2 providingparallel intersecting cylindrical bores for the reception of rotors l4and 16. At its ends the barrel part of the casing is closed by end covermembers l8 and 20, these latter members providing suitable bearings (notillustrated) for rotatably mounting the rotors within the casing. Therotors are interconnected by suitable timing gears for maintaining themin proper phase relation. Since this gearing forms no part of thepresent invention it is not illustrated.

The end cover 18 at the inlet end of thecompressor provides an end wall22 for closing the ends of the grooves in the rotors which form theworking spaces. The inlet end wall 22 is cut away to provide an inletport 24, a suitable outline for this port being indicated in Fig. 1 bythe line ab--c-cL-ej. Also, the lower portion of the member l8 and theadjacent portion Ho of the barrel l2 are shaped to provide a smoothlycontracting inlet surface 26 for admitting the fluid to be compressed tothe ends of the working spaces.

The inherent nature of the operation of the compressor is such that thedischarge or outlet port is located diagonally of the casing withrespect to the inlet port and in the present instance the outlet isindicated at 28. the port communicating with the working spacespartially in radial direction as indicated at 30, and partially in axialdirection as indicated at 32. Insofar as the present invention isconcerned, the specific outline of the exhaust port is not critical andhas not been shown. For a suitable outline of exhaust port, which it isto be noted is usually preferably of the combined radial and axialoutlet form, reference may be had to my previously granted patentshereinbefore referred to. s

The general form of compressor just described and its mode of operationare well known and need not be described herein in detail except insofaras the following factors of the operation are involved. The fluidinducted, which may be assumed to be air, enters the working spacesformed by the grooves through the inlet port and fills these groovesfrom end to end. The grooves are enclosed at their ends by therespective end walls of the casing, and as the rotors revolve so thatthe filled grooves pass out of communicationwith the inlet port, thetrapped volumes of air are carried peripherally, while as rotation ofthe rotors continues the intermeshing lands and grooves progressivelydecrease the volumes of the spaces or chambers formed in the groovesbetween the discharge ends of the rotors and the places where therespective grooves are closed by the cooperating lands. The places wherethe grooves are blocked or closed by the lands move progressively towardthe discharge end of the rotors as the latter revolve and compression iseffected in the spaces until they come into communication with theoutlet port. The number and pitch of the lands and grooves and the sizeand location of the outlet port will determine the compression ratio. Inthe example shown, one rotor has four lands while the other has six, thepitch of the lands on the two rotors being correspondingly different andthe rotors being geared to rotate at correspondingly different speeds.Insofar as this invention is concerned the number of lands on the tworotors may be the same or may have a difierent relationship. Ordinarilyit is desirable to effect compression within the working chambers, butif desired, the extent of the inlet and outlet ports for a given set ofrotors may be such that the grooves communicate with the outlet port atsubstantially the same time that they pass out of communie cation withthe inlet port so that the device will I operate merely as a positivedisplacement blower.

In the operation of a device of this kind, it is, of course, highlydesirable from a volumetric efficiency standpoint to fill the grooveswith the maximum weight of fluid, and to this end, when the device is ofthe high speed type involving high induction velocities, it is desirableto provide for essentially axial induction of air into the grooves. Forthis reason. an inlet port providing for substantially axial flow of airto the rotors such as herein disclosed is preferably employed. Also, itis to be noted that with substantially axial admission and high speedoperation, which results in relatively high inlet velocity, a definiteramming" 'efi'ect is obtainable since the high velocity axially flowingair flows throughthe grooves until it is stopped by impact against theend wall at the discharge end of the compressor. This setsup pressurewaves which travel back along the lengths of the respective grooves, andsince this takes an appreciable length of time, the inlet port 24 isadvantageously made of such extent peripherally that the port remains incommunication with the grooves for a predetermined length of time afterthey have been fully opened from end to end by the rotation of therotors. In order to illustrate this, the dotted line g--h indicates theposition which the end edge of the inlet port would have if the portclosed the grooves of the rotor M at the time when the grooves hadbeen'fully opened to the inlet port in a typical design, the port areaa-b--hg representing the amount of port area constituting what may betermed overlap for ramming purposes. Similarly, the area ef-i-7represents the port area for ramming overlap of the grooves of rotorfli.

While heretofore the fact has been recognized that throttling andrelated losses at the inlet due to acceleration to high velocity andchange in the direction of flow of the fluid into the grooves of therotors are encountered, the mag nitude of such losses has not heretoforebeen fully appreciated, nor has the nature of the causes of thisloss'heretofore been fully understood. Such losses have been reduced tosome extent by proper formation of the channel leading to the inletport, but I have discovered that even with the most favorableconfiguration of inlet port passages from a flow standpoint, arelatively large induction loss remains which can be materially reducedby properly modifying the inlet ends of the rotor lands.

In the type of device illustrated herein, in which axial compressionagainst an end wall is effected, two basic rotor characteristics areinvolved. The first is that the major portion of the lands of one rotor,which is conveniently referred to as the male rotor, lie outside thepitch circle of the rotor while the major portion of the depth of thelands of the other rotor, which is con-- veniently referred to as thefemale rotor lieinside the pitch circle of the rotor, the terms male andfemale being employed to distinguish these types of rotors from the twinrotors of the Roots type, or the like, which are characterized by pitchcircles lying midway between the tips and roots of the lands. Also, inthe present type device, if unnecessary and useless rotor lengths ofgiven rotor diameter are to be avoided, the helix angle of the lands andgrooves has a substantial value ordinarily lying 'within the range ofwhich the upper limit is of the order of 45 and the lower limit of whichis of the order of 25. Also, the higher the speed of operation of thedevice, the higher the helix angle is more likely to be the mostsuitabledesign.

The substantial helix angle employed creates'a dynamic induction lossdue to the flow of fluid from the inlet into the rotor grooves which apparently can be affected only to a certain extent by design of the inletport passage. Ishave discovered, however, that this loss can be verysubstantially reduced, and to an entirelyunexpected extent, bychamfering the edges of the rotor lands at their inlet ends, preferablyin the manner now to be'described. In the example illustrated, the malerotor i4 is provided with lands 34 defining between them grooves 36,while the female rotor is provided with lands 38 defining between themgrooves 40. The lands 34 comprise convexly curve'd sidesor flanks 34aand 34b, and have transversely extending inlet end surfaces 34c, whilethe lands transverse end surfaces 380. The profiles of the lands 34 and38 are preferably, but not necessarily, as disclosed in my prior PatentNo. 2,174,522, previously mentioned. In accordance with this inventionthe lands of the female rotor are chamfered along the lines of juncturebetween the leading flanks 38a and the inlet end surfaces 380, asindicated in Figs. 1, 3 and 4. Preferably these chamfers are formed byrounding to a convex cross section as indicated at 42.

. and further the chamfering is not necessarily carried to the apex ofthe land, so that the apexedges 44 of the leading flanks extend withoutchange of helix angle up to the inlet end surface of the land as seen inFig. 4. The reason for this is that if the chamfer is continued to theapex of the land so that the edge 44 is substantially rounded off orbeveled at the inlet end, an opening introducing a leakage loss will beopened up in certain positions of the rotors.

In other instances where extremely high speed operation is involved, thechamfering of the inlet ends of the rotor lands may be carried to theapex of the land, as indicated at 42a in Fi 5, but in order to avoidlosses of undesirable magnitude the apex edges of the leading flanks ofthe lands should extend to or closely adjacent to the inlet end surfacesof the lands without change of helix angle.

As will be seen from Fig. 1 the preferred construction results in theformation of generally crescent shaped chamfers having less lateralextent at the apexes and roots of the lands than at intermediate points.

Advantageously, the lands 34 of rotor N are also chamfered as shown inFigs. 1, 6 and "l, by rounding, as shown at 46 in Fig. 6, with thechamfering reduced toward the apex of the land so that the apex edge 48extends without change of helix angle up to the inlet end surface of therotor. As in the case of the chamfering of the female rotor, thespecific conditions of operation of the device may be such that it willbe desirable to carry the chamfering 46 to the apex line 48 of the landas shown in Fig. 8.

Also in some instances it may be desirable to chamfer along the lines ofjuncture of both the leading and the trailing flanks of the lands withthe inlet end surfaces thereof as shown in Figs. 9 and 10 respectively.In Fig. 9 the lands 3!! of the female rotor are shown as having chamfers42 joining the leading flanks with the inlet end surfaces and chamfers50 joining the trailing flanks with the inlet end surfaces. In Fig. 10the male land 34 is shown as with its leading fl'nk chamfered at 46 andwith its trailing flanks chamfered at 52. Experience has shown that fromthe standpoint of the extent to which induction loss is reduced bychamfering at the inlet ends of the rotor lands the maximum effect isobtained by chamfering of the leading flanks of the female rotor lands,which is the most important modification of the form of the land tomake. Chamfering of the leading flank of the male lands has a furtherloss reducing effect, but this appears to be not of the same order ofmagnitude as the effect of chamfering the leading flank of the femaleland at the inlet.

As noted above. the chamfering may be carried around the perimeter ofthe grooves so that both the leading and trailing flanks of the landsare chamfered, but, the chamfering of the trailing flanks of the landsappears to be relatively unimportant as compared with the chamfering ofthe leading flanks.

While but one form of device has been described forms of structures, andit is accordingly to be understood that the scope of the inventionincludes all structures falling within the terms of the appended claims.

What is claimed:

1. A rotary device comprising a casing and intermeshlng male and femalerotors mounted for rotation in said casing, said rotors being providedwith helical lands and grooves, the lands of the male rotor havingconvexly curved flanks at least the major portions of which lie outsidethe pitch circle of the rotor, the lands of the female rotor havingconcavely curved flanks at least the major portions of which lie insidethe pitch circle of-the rotor and the lands of both of said rotorshaving inlet end surfaces extending transversely of the axes of therotors, said casing having inlet and outlet ports and including a portedinlet end wall cooperating with said inlet end surfaces of the lands toalternately open and close the inlet end of each of said grooves as therotors revolve, the inlet end surface and the leading flank of each landof at least one of said rotors being joined by a chamfer extendinggenerally transversely of the .axis of rotation of the rotor along thecurved line of juncture of said inlet end surface with said leadingflank and the apex edge of said leading flank extending without changeof helix angle from a pointon the rotor remote from the inlet end of therotor to or closely adjacent to the inlet end surface of the land.

2. A rotary device as set forth in claim 1 in which said chamfers are;provided in the female rotor.

3. A rotary device as set forth in claim 1 in which said chamfers areprovided in both the male and female rotors.

4. A rotary device as set forth in claim 1 in which said chamfers arevof generally crescent shape having less lateral extent at the root andapex portionsof the.lands than at the radially intermediate portions ofthe lands.

5. A rotary device as set forth in claim 1 in which saidcasing isprovidedwith an inlet passage for directing fluid through said portedinlet end wall into said grooves essentially in axial direction and saidinlet port has a peripheral extent providing communication between saidpassage and said grooves until after the rotors have reached positionsof rotation in which the grooves are opened up to their outlet ends.

6. A rotor for the kind of device described having helical lands andgrooves and transversely extending inlet and outlet end surfaces, saidlands having concavely curved flanks at least the major portions ofwhich lie inside the pitch circle of the rotor and inlet end surfacesforming portions of the transversely extending inlet end surface of therotor, the inlet end surface and the leading flank of each land,considered in the direction in which the rotor is intended to turn,being joined by a chamfer extending generally transversely of the axisof rotation of the rotor along the curved line of juncture of said inletend surface with said leading flank and the apex edge of said leadingflank extending without change of helix angle from a point on the rotorremote from the inlet end of the rotor to or closely adjacent to theinlet end surface of the land.

circle of the rotor and inlet end surfaces forming portions of thetransversely extending inlet end surface of the rotor, the inlet endsurface and the leading flank of each land, considered in the directionin which the rotor is intended to turn, being joined by a chamfverextending generally transversely of the axis of rotation of the rotoralong the curved line of juncture of said inlet end surface with saidleading flank and the apex edge of said leading flank extending withoutchange of helix angle from a point on the rotor remote from the inletend' of the rotor to or closely adjacent to the inlet end surface of theland.

ALF LYSHOLM.

8 nnrsnancns man The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 885,194 Sponable Apr, 21. 1908935,328 Russell, H. M. Jr. Sept. 28. 1 09 1,144,184 Faust June 22, 19151,491,481 Huetter Apr. 22, 1924 1,762,708 Allred June 10, 1930 1,934,89Stelzer Nov. 14, 1933 2,111,568 Lysholm et al. Mar. 22, 1938 2,111,883Burghauser Mar. 22. 1938 2,321,696 Montelius June 15, 1943 2,325,617Lysholm Aug. 3, 1943 FOREIGN PATENTS Number Country Date 464,475 GreatBritain Apr. 18, 1937 464,493 Great Britain Apr. 16, 1937 France Feb.12, 1934

